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Senior Debt - Additional Information (Detail) - USD ($)
Sep. 25, 2015
Dec. 31, 2017
Dec. 31, 2016
Unused Lines of Credit [Member]      
Debt Instrument [Line Items]      
Unused lines of credit   $ 169,000,000  
LFB [Member] | Unused Lines of Credit [Member]      
Debt Instrument [Line Items]      
Unused lines of credit   18,000,000  
Amended and Restated Credit Agreement [Member]      
Debt Instrument [Line Items]      
Senior revolving credit facility $ 150,000,000    
Duration of senior revolving credit facility, in years 5 years    
Expiration of credit facility 2020-09    
Outstanding credit facility   $ 0 $ 0